The present invention generally relates to image display units and methods of producing same. More specifically, the present invention relates to an image display unit including light emitting devices and methods of producing same.
Various lightweight and thin display units have been developed, including LED (Light Emitting Diode) displays, liquid crystal displays, and plasma displays. The application of these image display units has been extended along with progress of computer techniques. For example, display units having a diagonal size of about 30 to 150 cm have been used for television receivers, video reproducing devices, and output units for game devices, and further, display units having a diagonal size smaller than about 30 cm have been used for vehicle-mounted navigation systems, picture recording systems, and monitors.
Each of the above-described image display units, however, has drawbacks in terms of characteristics such as resolution, luminance, light output/power efficiency, and image quality, and further, in terms of screen size and production costs. For example, in display units of a type using light emitting diodes arrayed in a matrix, individual light emitting diodes are collectively mounted to form an array of the light emitting devices. However, since each light emitting diode is packaged, it has a size as large as several millimeters. In general, a set of light emitting diodes of differing colors (e.g. red, green and blue) make up a pixel, which is the basic unit of the composition of an image on an image display unit. Thus, the size of each pixel becomes large, thereby degrading the resolution (i.e., the smaller the pixel size, the higher and better the resolution). Additionally, since the cost per each pixel is raised, the production costs become high, in particular, for an image display unit having a large screen.
In liquid crystal display units, a substrate forming part of the display unit is put in a film formation apparatus, kept in vacuum and devices such as transistors and wiring are formed using photolithography. In such display units, particularly, when increasing the resolution of the liquid crystal display, a process control must be performed on the order of μm. Accordingly, to improve the production yield, the process control must be strictly performed, and therefore, the production costs are increased when producing a liquid crystal display unit having a large screen. Further, liquid crystal display units have a viewing angle dependence in which the contrast or tint varies depending on a viewing angle, and also experience a response speed delay when one color is changed to another color.
Plasma display units primarily function by making use of a mechanism that generates a discharge in a narrow space on the order of a pixel size and visual light is generated by exciting phosphors with the aid of ultraviolet rays derived from an ionized gas generated from the discharge. Accordingly, plasma display units have low luminous efficiency and large power consumption. Further, external light is reflected from phosphors, thereby degrading the contrast. Additionally, plasma display units have a narrow color reproduction range.
Accordingly, each of the above-described image display units make it difficult to form a large-sized screen, are high in production costs, and have problems in terms of resolution, process control, image quality, and luminous efficiency.
Production costs for image display units using LEDs can be reduced by producing a number of LEDs from one wafer. More specifically, for example, the production costs of an image display unit can be reduced by separating an LED chip having a larger area into LED chips each having a significantly smaller area and mounting the LED chips, thus separated, on a board.
In this regard, there are various techniques known in which devices formed at a high density are moved to a wide region while being spaced from each other by transfer or the like, to obtain a relatively large display unit such as an image display unit. For example, U.S. Pat. No. 5,438,241 discloses a thin film transfer method, and Japanese Patent Laid-open No Hei 11-142878 discloses a method of forming a transistor array panel for a display unit.
In the transfer method disclosed in U.S. Pat. No. 5,438,241, devices densely formed on a substrate are coarsely re-arrayed on a specific display panel by transferring the devices densely formed on the substrate to a stretchable board provided with an adhesive layer, extending the stretchable board along a first axis and then along an orthogonal axis while monitoring the spacing between the devices along both axes, and transferring the devices on the extended stretchable board onto the display panel.
In the technique disclosed in Japanese Patent Laid-open No. Hei 11-142878, thin film transistors forming a liquid crystal display portion on a first substrate are all transferred on a second board, and the thin film transistors are selectively transferred from the second board to a third board with an array pitch corresponding to a pixel pitch (i.e., the distance from center to center of any two adjacent pixels).
The above-described techniques, however, encounter the following problems. The transfer method disclosed in U.S. Pat. No. 5,438,241, in which devices closely formed on a substrate are coarsely re-arrayed on a display panel requires that the device position is deviated by a chip site (e.g., about 20 μm), at a minimum, depending on at which position of an adhesively bonding surface of the device chip, a fixed point (supporting point) at the time of extension of the stretchable board is located. As a result, this transfer method requires accurate positional control for each device chip. Accordingly, when forming a high definition TFT array panel requiring positional accuracy of at least about 1 μm, it takes a lot of time to perform positioning of the TFT device chips including positional measurement and control for each TFT device chip. Another disadvantage of this transfer method is that when transferring TFT device chips on a resin film having a large thermal expansion coefficient, positional accuracy may be reduced depending on variations in temperature and stress, both before and after the positioning operation. Thus, from the viewpoint of mass-production, this transfer method has problems in terms of positional accuracy and time constraints.
The technique disclosed in Japanese Patent Laid-open No. Hei 11-142878 has the following problem. In this method, wiring electrodes and the like are formed after final transfer. However, since it has been required to reduce sizes of devices such as thin film transistors or light emitting devices for satisfying a requirement toward high integration of the devices so as to realize high-speed operation and reduction in costs, if a wiring layer and the like are formed after the devices are arrayed with an array pitch corresponding to a specific pixel pitch, then it is required to form wiring in a state that the micro-chips are already arrayed in a wider region. As a result, this method has a problem in terms of possible wiring failures due to problems with the positional accuracy of the devices.
There have been known some image display units in which light emitting devices such as light emitting diodes are mounted so as to be arrayed on a wiring board in a matrix. Japanese Patent No. 2895566 and Japanese Patent Laid-open No. Hei 9-293904 disclose light emitting diodes of a so-called flip-chip type. When producing an image display unit by arraying such light emitting diodes in a matrix, each light emitting diode must be contained in a package and an array of a number of these light emitting diodes must be mounted for assembly into a flat type image display unit or the like. Thus, since LEDs formed on a wafer are separated into individual chips and are each sealed in a package, each LED chip in a bare chip state has a size less than about 1 mm (e.g., each side of an approximately square-shaped chip is less than about 1 mm) and the package of the LED chip has a size on order of about several millimeters. As a result, the size of one pixel becomes large, thereby resulting in resolution degradation, and failing to produce a small-sized high definition image display unit. Further, for a light emitting diode made from a GaN based nitride semiconductor, since the light emitting diode is generally formed on a sapphire substrate, the package of each LED becomes thicker than the thickness of the sapphire substrate.
In view of the foregoing, a need exits to provide an image display unit capable of enhancing characteristics such as resolution, image quality, and luminous efficiency, facilitating formation of a large-sized screen, and reducing the production time and costs. An additional need exists to provide a method of arraying devices, which is capable of transferring micro-devices to a wider region without degrading positional accuracy after transfer and without the occurrence of a wiring failure.